The present invention relates to a filter cassette, and more particularly to a self-contained personal filter cassette for the sampling of substances in the air.
In many instances, airborne substances are collected for the purpose of assessing the potential risk to the health of persons arising from the inhalation of such substances at their places of work. Over the years, a wide variety of approaches have been suggested for collecting such samples. The physical act of sampling airborne substances generally involves the removal of a portion of the surrounding air by suction, and the analysis of the airborne substances thus aspirated. The airborne substances so collected are analyzed for concentration, composition and size distribution and such information is extremely useful in health hazard analysis and air pollution studies.
At present, two general types of filter cassettes are used for obtaining samples of airborne substances for subsequent analysis. These are the closed-face or in-line cassette and the open-face cassette. The open-face cassette has an inherent disadvantage in that it can easily be tampered with by the curious wearer. Additionally, the open-face cassette is easily exposed to splash and other interference from energetic processes, and it is easily contaminated when hung, facing down, directly next to a worker's clothing. While the closed-face cassette is not as easily tampered with or contaminated, as is the open-face cassette, the airborne substances collected on the filter medium in this cassette are unevenly distributed. The substances collected on the filter tend to accumulate directly behind the intake orifice thereby preventing certain tests from being performed on the sample. For example, analyses, such as polarized light microscopy, X-ray diffraction, and X-ray fluorescence analysis, cannot be done directly on the filter medium. Also, during gravimetric analysis, some of the airborne substances collected are easily dislodged and possibly lost from the filter medium surface since the substances tend to cake on the filter in one small area. Accordingly, there is real need for a filter cassette which collects uniform distributions of the airborne substances but which is free of the deficiencies inherent in the totally open-face cassette.
U.S. Pat. Nos. 3,693,410, 3,957,469 and 3,983,743 describe filter cassettes known in the art.